Low density foamed articles and methods for making

ABSTRACT

Foamed thermoplastic elastomeric polyurethane and ethylene-vinyl acetate copolymer articles are made with from about 0.1 to about 4 weight percent of supercritical fluid nitrogen based on polymer weight and from about 0.1 to about 5 weight percent of a supercritical fluid carbon dioxide based on polymer weight, with the supercritical fluid nitrogen and the supercritical fluid carbon dioxide being separately added to the molten polymer.

FIELD OF THE INVENTION

The present invention relates to foamed articles of thermoplastic elastomers and methods for making them.

INTRODUCTION TO THE DISCLOSURE

This section provides information helpful in understanding the invention but that is not necessarily prior art.

Thermoplastics are desirable as recyclable materials. However, thermoset materials can have properties better suited for some applications.

Brant et al., U.S. Pat. No. 6,759,443 describes polyurethane foam shoe soles made by foaming a polyurethane made from vinyl polymer-grafted polyoxyalkylene polyether. Polyethylene wax and polytetrafluoroethylene are added to improve abrasion resistance.

Takemura et al., U.S. Pat. No. 6,878,753 describes shoe soles and midsoles made of a thermoset polyurethane foam. The foam is made by a process comprising mixing a polyol solution, which is previously prepared by mixing a polyol, with a catalyst, water and urea, a chain extender, and an additive as occasion demands, with a polyisocyanate compound with stirring in a molding machine; and injecting the resulting mixture into a mold and foaming the mixture. The density of a molded article of the polyurethane foam is said to be 0.15 to 0.45 g/cm³.

It can be important for cushioning materials to have as properties resiliency and high energy return, but thermoplastic elastomers providing such properties have generally produced foams of higher density than is desirable in certain applications.

SUMMARY OF THE DISCLOSURE

This section provides a general summary rather than a comprehensive disclosure of the full scope of the invention and of all its features.

Disclosed is a method for making a low density foamed article by combining a molten thermoplastic polyurethane elastomer or a thermoplastic elastomer ethylene-vinyl acetate copolymer with supercritical fluid nitrogen and supercritical fluid carbon dioxide. The supercritical fluid nitrogen and supercritical fluid carbon dioxide are added separately to the molten polymer. The mixture of polymer and supercritical fluids is injection molded or extruded to form a low density article. The foamed thermoplastic polyurethane elastomer or ethylene vinyl acetate articles may have densities of as low as about 0.15 g/cm³. The method may be used to make very low density cushioning components for straps, protective gear, footwear components (such as a midsole or part of a midsole, an outsole, cushioning for a tongue or a sockliner), and for other applications.

The thermoplastic elastomer may also be combined with up to 15% by weight, based on polymer weight, of a physical or chemical blowing agent other than a supercritical fluid before the supercritical fluids are added.

The mold may contain a porous tool for absorbing gas generated during foaming of the molded article.

“A,” “an,” “the,” “at least one,” and “one or more” are used interchangeably to indicate that at least one of the item is present; a plurality of such items may be present unless the context clearly indicates otherwise. All numerical values of parameters (e.g., of quantities or conditions) in this specification, including the appended claims, are to be understood as being modified in all instances by the term “about” whether or not “about” actually appears before the numerical value. “About” indicates that the stated numerical value allows some slight imprecision (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. In addition, disclosure of ranges includes disclosure of all values and further divided ranges within the entire range.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are described with reference to the drawings, in which:

FIGS. 1A and 1B show an injection molding system that may be used in one embodiment of the disclosed process at different stages during the molding cycle;

FIGS. 2A and 2B are exploded views of the injection mold and the clamping device of the injection molding system of FIGS. 1A and 1B;

FIG. 3 illustrates an extrusion system that may be used in an embodiment of the process;

FIG. 4 illustrates a multi-hole blowing agent feed orifice arrangement and extrusion screw that may be used in embodiments of the process;

FIG. 5 illustrates the work surface area and the platen of a mold that may be used in the disclosed injection molding process; and

FIG. 6 illustrates an embodiment of a mold that may be used in the in the disclosed injection molding process.

DETAILED DESCRIPTION

A detailed description of exemplary, nonlimiting embodiments follows.

The low density foamed article is made by foaming a thermoplastic polyurethane elastomer or a thermoplastic ethylene-vinyl acetate copolymer using both supercritical fluid nitrogen and supercritical fluid carbon dioxide, which are separately added to the molten polymer and preferably dissolved in the polymer.

Thermoplastic TPU and EVA Elastomers

The thermoplastic polyurethane elastomer may have a melt index (also called a melt flow index or melt flow rate) of from about 5 to about 100 grams/10 min. (at 190° C., 8.7 kg) or from about 180 to about 300 grams/10 min. (at 200° C., 21.6 kg) as measured using the procedure of ASTM D1238. The thermoplastic ethylene-vinyl alcohol copolymer may have a melt index of from about 0.5 to about 50 grams/10 min. (at 190° C., 2.16 kg) as measured using the procedure of ASTM D1238. In various embodiments, the melt index for the polyurethane is preferably from about 5 to about 50 grams/10 min. (at 190° C., 8.7 kg), more preferably from about 15 to about 30 grams/10 min. (at 190° C., 8.7 kg) or preferably from about 180 to about 250 grams/10 min. (at 200° C., 21.6 kg), more preferably from about 180 to about 220 grams/10 min. (at 200° C., 21.6 kg) as measured using the procedure of ASTM D1238. In various embodiments, the melt index for the thermoplastic ethylene-vinyl alcohol copolymer is preferably from about 2.5 to about 10 grams/10 min. (at 190° C., 2.16 kg), more preferably from about 1 to about 10 grams/10 min. (at 190° C., 2.16 kg) as measured using the procedure of ASTM D1238.

Thermoplastic polyurethanes can be produced via reaction of diisocyanates with difunctional compounds reactive toward isocyanates. In general, the difunctional compounds have two hydroxyl groups (diols) and may have a molar mass of from 62 (the molar mass of ethylene glycol) to about 10,000, although difunctional compounds having other isocyanate-reactive groups (e.g., secondary amines) may be used, generally in minor amounts, and a limited molar fraction of tri-functional and mono-functional isocyanate-reactive compounds may be used. Preferably, the polyurethane is linear. Including difunctional compounds with molar masses of about 400 or greater introduces soft segments into the polyurethane. An increased ratio of soft segments to hard segments in the polyurethane causes the polyurethane to become increasingly more flexible and eventually elastomeric.

The elastomeric thermoplastic polyurethanes may be thermoplastic polyester-polyurethanes or thermoplastic polyether-polyurethanes. Nonlimiting, suitable examples of these include polyurethanes polymerized using as diol reactants polyesters diols prepared from diols and dicarboxylic acids or anhydrides, polylactone polyesters diols (for example polycaprolactone diols), polyester diols prepared from hydroxy acids that are monocarboxylic acids containing one hydroxyl group, polytetrahydrofuran diols, polyether diols prepared from alkylene oxides or combinations of alkylene oxides, and combinations of these. The elastomeric thermoplastic polyurethane may be prepared by reaction of at least one of these polymeric diols (e.g., polyester diol, polyether diol, polylactone diol, or polytetrahydrofuran diol), one or more polyisocyanates, and, optionally, one or more monomeric chain extension compounds. Chain extension compounds are compounds having two or more functional groups, preferably two functional groups, reactive with isocyanate groups. Preferably the elastomeric thermoplastic polyurethane is substantially linear (i.e., all or substantially all of the reactants are di-functional).

Nonlimiting examples of polyester diols used in forming the elastomeric thermoplastic polyurethane include those prepared by the condensation polymerization of dicarboxylic compounds, their anhydrides, and their polymerizable esters (e.g. methyl esters) and diol compounds. Preferably, all of the reactants are di-functional, although small amounts of mono-functional, tri-functional, and higher functionality materials (perhaps up to a few mole percent) can be included. Suitable dicarboxylic acids include, without limitation, glutaric acid, succinic acid, malonic acid, oxalic acid, phthalic acid, hexahydrophthalic acid, adipic acid, maleic acid, anhydrides of these, and mixtures thereof. Suitable polyols include, without limitation, wherein the extender is selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, cyclohexanedimethanol, 2-ethyl-1,6-hexanediol, 1,4-butanediol, 1,5-pentanediol, 1,3-propanediol, butylene glycol, neopentyl glycol, and combinations thereof. Small amounts of triols or higher functionality polyols, such as trimethylolpropane or pentaerythritol, are sometimes included. In a preferred embodiment, the carboxylic acid includes adipic acid and the diol includes 1,4-butanediol. Typical catalysts for the esterification polymerization are protonic acids, Lewis acids, titanium alkoxides, and dialkyl tin oxides.

Hydroxy carboxylic acid compounds such as 12-hydroxystearic acid may also be polymerized to produce a polyester diol. Such a reaction may be carried out with or without an initiating diol such as one of the diols already mentioned.

Polylactone diol reactants may also be used in preparing the elastomeric thermoplastic polyurethanes. The polylactone diols may be prepared by reacting a diol initiator, e.g., a diol such as ethylene or propylene glycol or another of the diols already mentioned, with a lactone. Lactones that can be ring opened by an active hydrogen such as, without limitation, ε-caprolactone, γ-caprolactone, β-butyrolactone, β-propriolactone, γ-butyrolactone, α-methyl-γ-butyrolactone, β-methyl-γ-butyrolactone, γ-valerolactone, δ-valerolactone, γ-decanolactone, δ-decanolactone, γ-nonanoic lactone, γ-octanoic lactone, and combinations of these can be polymerized. The lactone ring can be substituted with alkyl groups of 1-7 carbon atoms. In one preferred embodiment, the lactone is ε-caprolactone. Useful catalysts include those mentioned above for polyester synthesis. Alternatively, the reaction can be initiated by forming a sodium salt of the hydroxyl group on the molecules that will react with the lactone ring.

In preparing a polyether diol, a diol initiator such as ethylene glycol, propylene glycol, 1,4-butanediol, or another of the diols mentioned above is reacted with an oxirane-containing compound to produce a polyether diol. The oxirane-containing compound is preferably an alkylene oxide or cyclic ether, and more preferably it is a compound selected from ethylene oxide, propylene oxide, 1-butene oxide, tetrahydrofuran, and combinations of these. Other useful cyclic ethers that may be polymerized include, without limitation, 1,2-cyclohexene oxide, 2-butene oxide, 1-hexene oxide, tert-butylethylene oxide, phenyl glycidyl ether, 1-decene oxide, isobutylene oxide, cyclopentene oxide, 1-pentene oxide, and combinations of these. The polyether polymerization is typically base-catalyzed. The polymerization may be carried out, for example, by charging the hydroxyl-functional initiator and a catalytic amount of caustic, such as potassium hydroxide, sodium methoxide, or potassium tert-butoxide, and adding the alkylene oxide at a sufficient rate to keep the monomer available for reaction. Two or more different alkylene oxide monomers may be randomly copolymerized by coincidental addition and polymerized in blocks by sequential addition.

Tetrahydrofuran may be polymerized by a cationic ring-opening reaction using such counterions as SbF₆ ⁻, AsF₆ ⁻, PF₆ ⁻, SbCl₆ ⁻, BF₄ ⁻, CF₃SO₃ ⁻, FSO₃ ⁻, and ClO₄ ⁻. Initiation is by formation of a tertiary oxonium ion. The polytetrahydrofuran segment can be prepared as a “living polymer” and terminated by reaction with the hydroxyl group of a diol such as any of those mentioned above.

The polymeric diol, such as the polymeric polyester diols and polyether diols described above, that are used in making an elastomeric thermoplastic polyurethane synthesis preferably have a number average molecular weight (determined for example by the ASTM D-4274 method) of from about 300 to about 8,000, or from about 300 to about 5000, or from about 300 to about 3000.

The synthesis of an elastomeric thermoplastic polyurethane may be carried out by reacting one or more of the polymeric diols, one or more compounds having at least two (preferably two) isocyanate groups, and, optionally, one or more chain extension agents. The elastomeric thermoplastic polyurethanes are preferably linear and thus the polyisocyanate component preferably is di-functional or substantially di-functional. Useful diisocyanate compounds used to prepare the elastomeric thermoplastic polyurethanes, include, without limitation, methylene bis-4-cyclohexyl isocyanate, cyclohexylene diisocyanate (CHDI), isophorone diisocyanate (IPDI), m-tetramethyl xylylene diisocyanate (m-TMXDI), p-tetramethyl xylylene diisocyanate (p-TMXDI), ethylene diisocyanate, 1,2-diisocyanatopropane, 1,3-diisocyanatopropane, 1,6-diisocyanatohexane (hexamethylene diisocyanate or HDI), 1,4-butylene diisocyanate, lysine diisocyanate, 1,4-methylene bis-(cyclohexyl isocyanate), 2,4-tolylene (“toluene”) diisocyanate and 2,6-tolylene diisocyanate (TDI), 2,4′-methylene diphenyl diisocyanate (MDI), 4,4′-methylene diphenyl diisocyanate (MDI), o-, m-, and p-xylylene diisocyanate (XDI), 4-chloro-1,3-phenylene diisocyanate, naphthylene diisocyanates including 1,2-naphthylene diisocyanate, 1,3-naphthylene diisocyanate, 1,4-naphthylene diisocyanate, 1,5-naphthylene diisocyanate, and 2,6-naphthylene diisocyanate, 4,4′-dibenzyl diisocyanate, 4,5′-diphenyldiisocyanate, 4,4′-diisocyanatodibenzyl, 3,3′-dimethoxy-4,4′-biphenylene diisocyanate, 3,3′-dimethyl-4,4′-biphenylene diisocyanate, 1,3-diisocyanatobenzene, 1,4-diisocyanatobenzene, and combinations thereof. Particularly useful is diphenylmethane diisocyanate (MDI).

Useful active hydrogen-containing chain extension agents generally contain at least two active hydrogen groups, for example, diols, dithiols, diamines, or compounds having a mixture of hydroxyl, thiol, and amine groups, such as alkanolamines, aminoalkyl mercaptans, and hydroxyalkyl mercaptans, among others. The molecular weight of the chain extenders may range from about 60 to about 400 g/mol. Alcohols and amines are preferred in some embodiments; diols are particularly preferred. Typical examples of useful diols that are used as polyurethane chain extenders include, without limitation, 1,6-hexanediol, cyclohexanedimethanol, 2-ethyl-1,6-hexanediol, 1,4-butanediol, ethylene glycol and lower oligomers of ethylene glycol including diethylene glycol, triethylene glycol and tetraethylene glycol; propylene glycol and lower oligomers of propylene glycol including dipropylene glycol, tripropylene glycol and tetrapropylene glycol; 1,3-propanediol, neopentyl glycol, dihydroxyalkylated aromatic compounds such as the bis(2-hydroxyethyl)ethers of hydroquinone and resorcinol; p-xylene-α,α′-diol; the bis(2-hydroxyethyl)ether of p-xylene-α,α′-diol; m-xylene-α,α′-diol and the bis(2-hydroxyethyl)ether; 3-hydroxy-2,2-dimethylpropyl 3-hydroxy-2,2-dimethylpropanoate; and mixtures thereof. Suitable diamine extenders include, without limitation, p-phenylenediamine, m-phenylenediamine, benzidine, 4,4′-methylenedianiline, 4,4′-methylenibis(2-chloroaniline), ethylene diamine, and combinations of these. Other typical chain extenders are amino alcohols such as ethanolamine, propanolamine, butanolamine, and combinations of these. Preferred extenders include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,6-hexanediol, and combinations of these.

In addition to di-functional extenders, a small amount of tri-functional extenders such as trimethylolpropane, 1,2,6-hexanetriol and glycerol, or mono-functional active hydrogen compounds such as butanol or dimethyl amine, may also be present. The amount of tri-functional extenders or mono-functional compounds employed would preferably be a few equivalent percent or less based on the total weight of the reaction product and active hydrogen containing groups employed.

The reaction of the polyisocyanate(s), polymeric diol(s), and, optionally, chain extension agent(s) is typically conducted by heating the components, generally in the presence of a catalyst. Typical catalysts for this reaction include organotin catalysts such as stannous octoate or dibutyl tin dilaurate. Generally, the ratio of polymeric diol, such as polyester diol, to extender can be varied within a relatively wide range depending largely on the desired hardness of the elastomeric thermoplastic polyurethane. For example, the equivalent proportion of polyester diol to extender may be within the range of 1:0 to 1:12 and, more preferably, from 1:1 to 1:8. Preferably, the diisocyanate(s) employed are proportioned such that the overall ratio of equivalents of isocyanate to equivalents of active hydrogen containing materials is within the range of 0.95:1 to 1.10:1, and more preferably, 0.98:1 to 1.04:1. The polymeric diol segments typically are from about 25% to about 65% by weight of the elastomeric thermoplastic polyurethane, and preferably from about 25% to about 50% by weight of the elastomeric thermoplastic polyurethane.

The polyurethanes may be used in any combination to make the low density foamed articles, including using combinations of thermoplastic polyether polyurethane elastomers and thermoplastic polyester polyurethane elastomers.

In certain embodiments it may be preferred to use an elastomer polyester polyurethane prepared from a polyester of a dicarboxylic acid having from 4 to about 8 carbon atoms, or its esterifiable derivatives such as anhydride or lower alkyl ester, and a diol having from about 4 to about 8 carbon atoms, particularly an unbranched diol. In certain embodiments, it may be preferred to use a polyester polyurethane prepared from a poly(ε-caprolactone) diol polyester or a polyether polyurethane prepared from a poly(tetrahydrofuran) polyether [also know as a poly(tetramethylene oxide) polyether or poly(tetramethylene ether) glycol]. These may be used in any combination.

In certain embodiments, preferred polyurethanes in the solid (unfoamed) form may have a Shore A hardness of from about 35 A to about 85 A, preferably from about 50 A to about 75 A, and more preferably from about 60 A to about 70 A, as measured by the method of according to ASTM D2240. In certain preferred embodiments, the polyurethane has a Shore A hardness of from about 50 A to about 80 A or from about 60 A to about 70 A and a melt flow index of from about 5 to about 100 grams/10 min. (at 190° C., 8.7 kg) or from about 180 to about 300 grams/10 min. (at 200° C., 21.6 kg)

Nonlimiting examples of suitable commercially available polyurethane elastomers include Elastollan® SP9213 (melt flow index of 200 g/10 min. (at 200° C., 21.6 kg), Elastollan® 1170, Elastollan® 1180, and a soft 45 A polyurethane version of these, which are available from BASF Polyurethanes GmbH; Texin® 1209, available from Bayer MaterialScience; and Estane MPD 00361B, available from the Lubrizol Corporation.

Elastomeric ethylene-vinyl acetate copolymers may be prepared by free-radical emulsion polymerization of ethylene and up to about 50% by weight vinyl acetate. The vinyl acetate monomer is usually at least about 10% by weight, preferably at least about 25% by weight of the monomers used. The ethylene-vinyl acetate copolymer has a vinyl acetate content of preferably from about 25 weight percent to about 50 weight percent and more preferably from about 35 weight percent to about 50 weight percent. The ethylene-vinyl acetate copolymers may have a melt flow index of from about 0.5 to about 50 grams/10 min. (at 190° C., 2.16 kg), preferably 2.5 to 10 grams/10 min. (at 190° C., 2.16 kg) as measured using the procedure of ASTM D1238. Nonlimiting examples of suitable commercially available ethylene-vinyl acetate copolymers include Elvax 265, Elvax 40L-3 from DuPont and Lavaprene 400 from Langxess. The ethylene-vinyl acetate copolymers may be used in combination.

Pellets, beads, particles, or other pieces of the polyurethane thermoplastic elastomer or ethylene-vinyl acetate copolymer (EVA) are introduced into an extruder along with a physical or chemical blowing agent other than a supercritical fluid, with the blowing agent being used in an amount up to about 15% by weight based on polymer weight. The blowing agent may preferably be one that contains or produces nitrogen or carbon dioxide. The pellets may have a regular or irregular shape, including generally spherical, cylindrical ellipsoidal, cubic, rectangular, and other generally polyhedral shapes as well as irregular or other shapes, including those having circular, elliptical, square, rectangular, or other polygonal cross-sectional outer perimeter shapes or irregular cross-sectional shapes with or without uniform widths or diameters along an axis. “Generally” is used here to indicate an overall shape that may have imperfections and irregularities, such as bumps, dents, imperfectly aligned edges, corners, or sides, and so on.

Blowing Agent (Other than a Supercritical Fluid)

Optionally, a blowing agent other than a supercritical fluid may be incorporated into the polymer pellets or may be dry blended with the pellets. In general, the blowing agent may be used in amounts of from about 1% by weight to about 15% by weight, based on the total weight of the polymer. In various embodiments, blowing agent or blowing agents other than supercritical fluids may be using in amounts of from about 2% by weight to about 15% by weight, or from about 3% by weight to about 12% by weight, or from about 3% by weight to about 10% by weight, based on the weight of the polymer.

Physical foaming and blowing agents function as gas sources by undergoing a phase change. Suitable physical blowing and foaming agents may be selected from the group consisting of aliphatic hydrocarbons and their chloro- and fluoro-derivatives. Typical foaming and blowing agents may be selected from isomers of pentane, hexane, heptane, fluorocarbons, trichlorofluoromethane, dichlorodifluoromethane, dichlorotetrafluoroethane, monochlorodifluoromethane, and methylene chloride. Chemical foaming and blowing agents produce a gas via a chemical reaction. Suitable chemical foaming and blowing agents may be selected from, for example, azo-type compounds for the generation of N₂, ammonium compounds that generate NH₃, and mixtures of carbonates and acids that generate CO₂. Specific suitable examples include sodium bicarbonate, dinitrosopentamethylene-tetramine, sulfonyl hydroxides, azodicarbonamide, p-toluenesulfonyl semicarbazide, 5-phenyltetrazole, diisopropylhydrazodicarboxylate and sodium borohydrite. The thermal decomposition of the foaming or blowing agents can be lowered through addition of activators or accelerators, as is known in the art.

In another embodiment, microbeads containing physical blowing agent are used as a blowing agent. The microbeads are usually made of a shell of thermoplastic polymer with, in the core, a liquid, low-boiling gas based on alkanes. The preparation of these microbeads is described by way of example in U.S. Pat. Nos. 3,615,972, 6,509,384, 7,956,096, and 8,388,809, the disclosures of which are incorporated herein by reference. The microbeads typically have a diameter of from 5 to 50 μm. Examples of suitable microbeads are obtainable with Expancell® from Akzo Nobel.

The chemical or physical blowing agents may be used in any combination. The blowing agent or combination of blowing agents other than supercritical fluids may be incorporated into the polymer pellets (as in the common procedure of forming a masterbatch), may be dry blended with the polymer pellets before being added to the extruder, or may be added to the extruder separately from the pellets.

Other materials that may be used in thermoplastic elastomeric polyurethane or ethylene-vinyl acetate copolymer compositions that are foamed include, without limitation, colorants, particularly pigments; fillers, including clays, nanoclays, nanotubes, halloysite clay, which is available from Applied Minerals under the trademark Dragonne™, and so on; surfactants, release agents, antioxidants, stabilitzers, crosslinkers that may be included in amounts that lightly crosslink the elastomer but leave the elastomer foam a thermoplastic, and so on.

Equipment and Process

In describing the equipment and process, “clamping force” refers to the force that holds together the two mold halves that define the mold cavity. The clamping force can be determined, for example, by multiplying the hydraulic pressure by the piston surface area in a clamping device (for hydraulic clamping devices), by measuring the force using a load cell associated with the mold halves and/or platens, or by measuring tie bar elongation or strain and converting to a clamping force. The term “injection pressure” refers to the pressure at which polymeric material is injected into the mold. The injection pressure can be determined, for example, by measuring the hydraulic load (for hydraulic injection systems) or by measuring the servomotor torque and multiplying it by the appropriate mechanical advantage factor of the system (for electrical injection systems). The term “supercritical fluid additive” refers to a supercritical fluid under temperature and pressure conditions within an extruder (e.g., 12, FIG. 1). The supercritical fluid additive may or may not be a supercritical fluid prior to introduction into the extruder (e.g., when supercritical fluid additive is in source 22, FIG. 1) and the supercritical fluid additive may be introduced into the polymeric material in the extruder in any flowable state, for example, as a gas, a liquid, or a supercritical fluid.

With reference now to the drawings, FIGS. 1A and 1B schematically illustrate an injection molding system 10 that may be used in one embodiment of the disclosed process. An extruder 12 of molding system 10 includes a polymer processing screw 14 that is rotatable within a barrel 16 to convey the polymeric material in a downstream direction 18 within a polymer processing space 20 defined between the screw and the barrel. A source 22 a of nitrogen supercritical fluid additive is connected via conduit 23 a to a port 24 a formed within the barrel. A source 22 b of carbon dioxide supercritical fluid additive is connected via conduit 23 b to a port 24 b formed within the barrel. Ports 24 a, 24 b are preferably spaced apart, particularly preferably located on opposite sides of extruder 12, preferably linearly spaced along direction 16 as well, so as to allow, as far as possible, the nitrogen supercritical fluid additive and the carbon dioxide supercritical fluid additive to independently be mixed into the molten polymer. While not wishing to be bound by theory, it is thought that separately adding the nitrogen supercritical fluid additive and the carbon dioxide supercritical fluid additive to the polymer improves even distribution of larger foam cells.

Extruder 12 includes an outlet 26 connected to an injection mold 28. Optionally, system 10 includes a control system 25 that is capable of controlling the injection molding process and supercritical fluid additive introduction into the polymeric material.

Generally, injection molding system 10 operates cyclically to produce multiple molded articles. At the beginning of a typical molding cycle, screw 14 is positioned at a downstream end 32 of barrel 16. The thermoplastic polyurethane elastomer or EVA, typically in pelletized form, and the optional blowing agent other than supercritical fluid, if used, and optionally other additives, are fed into polymer processing space 20 from a hopper 34 through an orifice 36. Barrel 16 is heated by one or more heating units 35 and screw 14 rotates to knead and mix the melting polyurethane elastomer or EVA and optional blowing agent and additives and to convey the polymer in downstream direction 18. The polyurethane elastomer or EVA should be in a fluid state at the point of supercritical fluid additives are introduced through ports 24 a, 24 b. The flow rate of supercritical fluid additives into the polymer mixture may be metered, for example, by a metering device 39 a positioned between source 22 a and port 24 a and by a metering device 39 b positioned between source 22 b and port 24 b. The metering devices 39 a, 39 b may also be connected to control system 25, which sends signals to control the respective supercritical fluid additive flow rates. The molten polymer and supercritical fluid additives are mixed and conveyed downstream by the rotating screw and accumulated in a region 38 within the barrel downstream of the screw. The accumulation of the mixture in region 38 creates a pressure that forces the screw axially in an upstream direction in the barrel. After a sufficient charge of the mixture has been accumulated, screw 14 ceases to rotate and stops moving in the upstream direction. Preferably, when the screw stops the introduction of supercritical fluid additive into the polymeric material is, or has been, stopped, for example, by the operation of an injector valve 40 associated with port 24.

Then, the screw is moved axially in a downstream direction by an injection device 42 to downstream end 32 of the barrel, returning to the original screw position to inject the accumulated charge of the mixture through outlet 26 of the extruder and into a cavity 44 (FIG. 2A) defined between mold halves 46 a, 46 b. A shut-off nozzle valve 45 associated with the outlet of the extruder typically is opened to permit the mixture to flow into the cavity. After the charge is injected into the cavity, valve 45 is typically closed. As described further below and shown in FIGS. 2A and 2B, a clamping device 48 holds mold halves 46 a, 46 b of the mold 28 (FIG. 2A) together during injection and the subsequent cooling of the polymeric material. After the polymeric material sufficiently solidifies, clamping device 48 separates mold halves 46 a, 46 b (FIG. 1B) to eject a molded article. The molding cycle is repeated to produce additional molded articles.

The extruder screw extrudes (FIG. 3) or injects (FIGS. 1A, 1B) the mixture at a preferred rate of from about 1 to about 5 inches per second (2.54 to about 12.7 cm/s). The mold may be heated, for example up to about 50° C. The injection pressure may be from about 9000 psi (62 MPa) to about 30,000 psi (207 MPa), preferably from about 18,000 psi (124.1 MPa) or from about 22,000 psi (152 MPa) to about 28,000 psi (193 MPa) or to about 30,000 psi (207 MPa), particularly preferably from about 18,000 psi (124.1 MPa) to about 28,000 psi (193 MPa).

Molding system 10 may include a number of variations from the illustrative embodiment as known to one of ordinary skill in the art. For example, a mold may define more than one cavity in which articles may be molded and may include a hot runner gate to introduce polymeric material into the cavities. The hot runner gate may also be provided with a valve to selectively control introduction of the polyurethane or EVA material. It should also be understood that the injection molding system may be a hydraulic system, an electrical system, or a hybrid hydraulic/electric system.

Control system 25 can coordinate the operation of metering devices 39 a, 39 b with screw 14 so that a desired amount of each of the supercritical fluid additives is introduced into the polymeric material to form a mixture having the desired weight percentage of each of the supercritical fluid additives. In some embodiments, a first controller controls the operation of the injection molding system and a second controller controls supercritical fluid additives introduction. In other embodiments, a single controller controls operation of the injection molding system and supercritical fluid additives introduction.

Referring to FIGS. 2A and 2B, injection mold 28 and clamping device 48 are shown. In the illustrative embodiment, mold half 46 a is secured to a movable platen 52 a, and mold half 46 b is secured to a fixed platen 52 b. Platen 52 a is slideably mounted on a plurality of tie bars 56 which extend from a backside 58 of system 10 to fixed platen 52 b. Platen 52 a reciprocates on tie bars 56 to open and close mold 28 in response to the action of clamping device 48 which is synchronized with the molding cycle. Mold 28 is closed when clamping device 48 pushes platen 52 a in the direction of arrow 60, which forces mold half 46 a against mold half 46 b (FIG. 2A). Clamping device 48 holds mold halves 46 a, 46 b together with a clamping force during injection and while the molded part cools. To open the mold, clamping device retracts platen 52 a in a direction opposite arrow 60 which separates mold halves 46 a, 46 b (FIG. 2B).

Other configurations of the injection mold and clamping device may also be used in the disclosed process. For example, in some cases, the mold may not include platens, but rather the movable mold half may be secured directly to the clamping device and the other mold half secured to the frame of the system. In other embodiments, a pressure measuring device may be associated with mold cavity 44 to monitor pressure within the mold (i.e., cavitation pressure). The pressure measuring device may, for example, access the mold cavity through a wall of one of the mold halves. The pressure measuring device can send output signals representative of the cavitation pressure, for example, to control system 25 to control various molding parameters such as injection speed and injection force.

Clamping device 48 may be any suitable type. Clamping device 48 may be hydraulically or mechanically/electrically powered. A clamping device can be characterized by the maximum force it is capable of providing. Suitable clamping devices may provide a maximum force, for example, of between about 10 tons-force (98 kN) and about 10,000 tons-force (98,000 kN), and more typically between about 50 tons-force (490 kN) and about 3,000 tons-force (about 29,420 kN). The specific clamping force depends on various factors, such as the article being molded.

Clamping device 48 generally needs to provide a clamping force sufficient to prevent polymeric material injected into cavity 44 from flashing between mold halves 46 a, 46 b.

As shown in FIGS. 5 and 6, mold back surface 59 at arrows 1 preferably includes a porous tool 220 in mold cavity 44 adjacent mold wall 210. The porous tool may be located anywhere adjacent an inner mold surface, such as along the back surface 59 as shown, a side surface 61, or a surface of the fixed mold half 46 b, adjacent more than one area of the inner mold surface, or adjacent all inner surfaces, The porous tool may be an insert that is generally the dimensions of the mold cavity, or may be one or a plurality of smaller inserts—for example, three to five inserts—spaced within the mold cavity. The porous tool, or all of the porous tools collectively placed in the mold, may have a pore size of from about 3 μm to about 20 μm, preferably from about 7 μm to about 12 μm, for absorbing gas generated during foaming of the molded article. The porous tool may be a porous metal or metal alloy, such as a porous aluminum or a porous steel, a porous ceramic, such as a porous cordierite, zirconium oxide, silicon carbide, aluminum oxide, or silicon nitride ceramic, or another porous material that does not melt or soften at the temperatures reached during the molding process. Open cell metal and ceramic foams or sintered porous metal materials, which can be made from metal powders, can be fabricated as a porous tool in a desired shape to fit in one or more areas inside the mold cavity and absorb gas during foaming of the polymer.

The porous tool may also have any desired shape and be placed at a desired location or locations within the mold cavity. For example, the porous tool may have a feature extending into the mold cavity.

In another embodiment shown in FIG. 3, the mixture of polyurethane or EVA, optional blowing agent or other additives, nitrogen supercritical fluid additive, and carbon dioxide supercritical fluid additive may be extruded as a foam instead of injection molded. An extrusion system 130 includes a screw 138 that rotates within a barrel 132 to convey, in a downstream direction 33, polymeric material in a processing space 135 between the screw and the barrel. The polymeric material is extruded through a die 137 fluidly connected to processing space 135 and fixed to a downstream end 136 of barrel 132. Die 137 is configured to form an extrudate 139 of microcellular foam in the desired shape. The thermoplastic polyurethane elastomer or EVA, for example in the form of pellets containing the blowing agent or as a dry mixture of polyurethane elastomer or EVA pellets and the blowing agent, is gravity fed into polymer processing space 135 through orifice 146 from a standard hopper 144. Extrusion screw 138 is operably connected, at its upstream end, to a drive motor 140 which rotates the screw. Positioned along extrusion barrel 132 are temperature control units 142, which for example can be electrical heaters or can include passageways for temperature control fluid, that can be used to heat the polyurethane or EVA within the extrusion barrel to facilitate melting, to cool the stream to control viscosity, skin formation, or dissolution of the nitrogen and carbon dioxide supercritical fluids. The temperature control units can operate differently at different locations along the barrel, that is, to heat at one or more locations, and to cool at one or more different locations. Any number of temperature control units can be provided. Temperature control units 142 can also optionally be used to heat die 137.

The nitrogen supercritical fluid additive is introduced into the polymer stream through a port 154 a in fluid communication with a source 156 a of the nitrogen supercritical fluid additive. Device 158 a can be used to meter the nitrogen supercritical fluid additive. Similarly, the carbon dioxide supercritical fluid additive is introduced into the polymer stream through a port 154 b in fluid communication with a source 156 b of the carbon dioxide supercritical fluid additive. Device 158 b can be used to meter the carbon dioxide supercritical fluid additive.

Supercritical Fluid

Supercritical CO₂ may be combined with the polymer in an amount of from about 0.1 weight percent to about 5 weight percent, preferably from about 0.5 weight percent to about 5 weight percent, and more preferably from about 1 weight percent to about 3 weight percent based on polymer weight. Supercritical N₂ may be combined with the polymer in an amount of from about 0.1 weight percent to about 4 weight percent, preferably from about 0.4 weight percent to about 2.5 weight percent, and more preferably from about 0.7 weight percent to about 1.5 weight percent based on polymer weight.

When forming microcellular materials, it may be preferable to form a single-phase solution of polymeric material and the two supercritical fluid additives before extruding or injection molding the polymer mixture. To aid in the formation of a single-phase solution, each of the supercritical fluids may be introduced through a plurality of ports 24 arranged in the barrel, though it should be understood that a single port may also be utilized for each of the supercritical fluid additives to form a single-phase solution. When multiple ports 24 are utilized, the ports can be arranged radially about the barrel or in a linear fashion along the length of the barrel. An arrangement of ports along the length of the barrel can facilitate injection of supercritical fluid additives at a relatively constant location relative to the screw when the screw moves axially (in an upstream direction) within the barrel as the mixture of polymeric material and supercritical fluid additives is accumulated. Where radially-arranged ports are used, ports 24 may be placed in equally-spaced positions about the extruder barrel, or in any other configuration as desired. Preferably, ports through which nitrogen supercritical fluid additive is introduced are spaced as far as practicable from ports through which carbon dioxide supercritical fluid additive is introduced to allow the two different supercritical fluid additives to separately dissolve or be mixed into the polymer.

Each of ports 24 a, 24 b (FIGS. 1A, 1B) may include a single orifice or a plurality of orifices. In the multi-orifice embodiments the port may include at least about 2, and some cases at least about 4, and others at least about 10, and others at least about 40, and others at least about 100, and others at least about 300, and others at least about 500, and in still others at least about 700 orifices. In another embodiment, each of ports 24 a, 24 b includes an orifice containing a porous material that permits supercritical fluid additive to flow through and into the barrel, without the need to machine a plurality of individual orifices for each of the supercritical fluid additives.

To further promote the formation of a single-phase solution, ports 24 a, 24 b may be located at a section of the screw that may include full, unbroken flight paths. In this manner, each flight, passes or “wipes” the port including orifices periodically, when the screw is rotating. This wiping increases rapid mixing of supercritical fluid additive and polymeric material in the extruder and the result is a distribution of relatively finely divided, isolated regions of supercritical fluid additive in the polymeric material immediately upon injection into the barrel and prior to any mixing. Downstream of ports 24 a, 24 b, the screw may include a mixing section which has highly broken flights to further mix the polymeric material and supercritical fluid additive mixture to promote formation of a single-phase solution.

FIG. 4 illustrates an embodiment of the extruder 130 of FIG. 3 in which the supercritical fluid additive ports are illustrated in greater detail. In this embodiment, ports 154 a, 154 b are located in the injection section of the screw at a region upstream from mixing section 160 of screw 138 (including highly-broken flights) at a distance upstream of the mixing section of no more than about 4 full flights, preferably no more than about 2 full flights, or no more than 1 full flight. Positioned in this way, the injected supercritical fluid additives are very rapidly and evenly mixed into the polymer melt to produce a single-phase solution of the supercritical fluids in the polymer melt.

Ports 154 a, 154 b, in the preferred embodiment illustrated, are multi-hole ports including a plurality of orifices 164 connecting the blowing agent sources with the extruder barrel. As shown, in preferred embodiments ports 154 a, 154 b are provided about the extruder barrel at positions radially separated and can be in alignment longitudinally with each other. A plurality of ports 154 a, 154 b for each of the supercritical fluid additives can be placed in spaced positions about the extruder barrel and along the extruder barrel, each including multiple orifices 164. In this manner, where each orifice 164 is considered a supercritical fluid additive orifice, there may be at least about 10, preferably at least about 40, more preferably at least about 100, more preferably at least about 300, more preferably at least about 500, and more preferably still at least about 700 supercritical fluid additive orifices for each of the nitrogen and carbon dioxide supercritical fluid additives in fluid communication with the extruder barrel.

Also in preferred embodiments is an arrangement (as shown in FIG. 4) in which the supercritical fluid additive orifice or orifices are positioned along the extruder barrel at a location where, when a preferred screw is mounted in the barrel, the orifice or orifices are adjacent full, unbroken flights 165. In this manner, as the screw rotates, each flight, passes, or “wipes” each orifice periodically. This wiping increases rapid mixing of blowing agent and fluid foamed material precursor by, in one embodiment, essentially rapidly opening and closing each orifice by periodically blocking each orifice, when the flight is large enough relative to the orifice to completely block the orifice when in alignment therewith. The result is a distribution of relatively finely-divided, isolated regions of blowing agent in the fluid polymeric material immediately upon injection and prior to any mixing.

Referring again to FIG. 3, a mixing section of screw 138, following the gas injection section, is constructed to mix the blowing agent and polymer stream to promote formation of a single phase solution of supercritical fluid additive and polymer. The mixing section includes unbroken flights which break up the stream to encourage mixing. Downstream the mixing section, a metering section builds pressure in the polymer-supercritical fluid additive stream prior to die 137. Die 137 can have any variety of configurations, as is well known in the art, to produce microcellular foam in specific forms, for example sheets or profiles. In addition to shaping extrudate 139, die 137 may also perform the function of nucleating the polymer and supercritical fluid additive single-phase solution. The pressure in the single phase solution drops as the solution flows through the internal passageways of the die. This pressure drop causes the solubility of the supercritical fluid additive in the polymer to decrease, which is the driving force for the cell nucleation process. The extent of pressure drop depends upon the dimensions of the passageway. Specifically the dimensions that effect pressure drop include the shape of the passageway, the length of the passageway, and the thickness of the passageway. Typically, the geometry of the die is designed to give a pressure drop suitable for cell nucleation to produce a microcellular foam. Other equipment (not illustrated) downstream of the die is used, as required, for additional shaping of the extrudate into a final form.

The foamed thermoplastic elastomeric polyurethane article may have a density of less than about 0.3 g/cm³, preferably less than about 0.25 g/cm³, more preferably less than about 0.2 g/cm³. In various embodiments, the foamed thermoplastic elastomeric polyurethane article may have a density of from about 0.15 to about 0.3 g/cm³, or a density of from about 0.15 to about 0.25 g/cm³, or a density of from about 0.15 to about 0.2 g/cm³.

The foamed thermoplastic elastomeric EVA article may have a density of less than about 0.3 g/cm³, preferably less than about 0.25 g/cm³, more preferably less than about 0.2 g/cm³. In various embodiments, the foamed thermoplastic elastomeric polyurethane article may have a density of from about 0.15 to about 0.3 g/cm³, or a density of from about 0.15 to about 0.25 g/cm³, or a density of from about 0.15 to about 0.2 g/cm³.

The shaped article may be of any dimensions. For example, the molded article may be sized as a cushion or cushioning element that can be included in an article of footwear, for example part of a footwear upper, such as a foam element in a collar or tongue, as an insole, as a midsole or a part of a midsole, or an outsole or a part of an outsole; foam padding in shinguards, shoulder pads, chest protectors, masks, helmets or other headgear, knee protectors, and other protective equipment; an element placed in an article of clothing between textile layers; in clothing, or may be used for other known padding applications for protection or comfort, especially those for which weight of the padding or cushioning is a concern. The molded article may be incorporated as cushioning into other articles.

In various embodiments, the molded article is a midsole for an article of footwear. A midsole provides cushioning in the footwear. A midsole should be durable but also preferably adds as little weight as possible to the footwear while still cushioning to the desired degree. A midsole also should be able to be bonded to an outsole, an upper, or any other components (e.g., a shank, an airbag, or decorative components) in making an article of footwear.

In other embodiments, the molded article is an outsole for an article of footwear.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention. 

1. A method for making a low density foamed article, comprising: combining in an extruder a molten polymer selected from the group consisting of thermoplastic polyurethane elastomers and thermoplastic ethylene-vinyl acetate copolymers with (a) from about 0.1 to about 4 weight percent of supercritical fluid nitrogen based on polymer weight and (b) from about 0.1 to about 5 weight percent of a supercritical fluid carbon dioxide based on polymer weight to form a mixture, wherein the supercritical fluid nitrogen and the supercritical fluid carbon dioxide are separately added to the molten polymer; and injection molding and foaming the mixture or extruding and foaming the mixture.
 2. A method according to claim 1, wherein the molten polymer comprises a thermoplastic polyurethane elastomer having a Shore A hardness of from about 35 A to about 85 A.
 3. A method according to claim 2, wherein the Shore A hardness is from about 50 A to about 80 A.
 4. A method according to claim 1, wherein the molten polymer comprises a thermoplastic ethylene-vinyl acetate copolymer having from about 25 weight percent to about 50 weight percent vinyl acetate content.
 5. A method according to claim 1, wherein a physical or chemical blowing agent other than a supercritical fluid is combined with the molten polymer in an amount of up to about 15 weight percent based on polymer weight.
 6. A method according to claim 1, wherein from about 1 to about 3 weight percent of supercritical CO₂ based on polymer weight and from about 0.4 to about 2.5 weight percent of supercritical N₂ based on polymer weight are added to the molten polymer.
 7. A method according to claim 1, wherein the mixture further comprises a crosslinker.
 8. A method according to claim 1, wherein the mixture is injection molded at a rate of from about 1 to about 5 inches per second.
 9. A method according to claim 1, wherein mixture is injection molded into a mold containing a porous tool.
 10. A method according to any of claim 9, wherein the porous tool is a porous metal or metal alloy.
 11. A method according to claim 9, wherein the porous tool is an insert that is generally the dimensions of the mold cavity.
 12. A method according to claim 9, wherein the porous tool comprises a plurality of inserts spaced within the mold cavity.
 13. A method according to claim 1, wherein the molten polymer is selected from the group consisting of thermoplastic polyurethane elastomers having a melt index of from about 5 to about 100 grams/10 min. (at 190° C., 8.7 kg) or from about 180 to about 300 grams/10 min. (at 200° C., 21.6 kg) and thermoplastic ethylene-vinyl acetate copolymers having a melt index of from about 0.5 up to about 50 grams/10 min. (at 190° C., 2.16 kg).
 14. A method according to claim 1, wherein a blowing agent other than a supercritical fluid is combined with the molten polymer in an amount of from about 3% by weight to about 12% by weight based on polymer weight.
 15. A method according to claim 1, wherein the mixture is injection molded at an injection pressure of from about 110 MPa to about 207 MPa in a mold comprising a porous tool.
 16. A foamed article made by a method according to claim
 1. 17. A foamed article according to claim 16, wherein the article has a density of from about 0.15 to about 3 g/cm³.
 18. A foamed article according to claim 16, wherein the article is a cushion or cushioning element for an article of footwear, protective equipment, or clothing. 